Distillation devices generally convert a liquid, for instance water or ethyl alcohol, to a vapor, transport the vapor to a position remote from its source, and convert the vapor back to its liquid state. The liquid that has been so distilled, i.e., the distillate, is thereby separated from substances that do not vaporize under the conditions used to vaporize the liquid in question. Such non-vaporizable substances can be considered "impurities" for some purposes, but often such impurities are themselves important products of the distillation process. Distillation is essentially a separation process and the relative value of separated products does not alter this essential characteristic.
Water is typically distilled to separate it from dissolved solids, such as salts or the like. Alcohol is typically distilled to separate it from the solid matter of the mash in which the alcohol is brewed. In either typical case, of course, the salts or mash, which are freed from at least a substantial portion of the water or alcohol, can be a valuable distillation product. It is, however, the water or alcohol distillate that is recovered free of other components of the water or alcohol source. For simplicity, therefore, the other substances present in the source of the distillate will be referred to hereinafter as impurities.
All distillation processes have a net energy requirement. Energy is required to convert the liquid in the source material to its vapor state. Although energy is released upon condensation of the vapor to a liquid, it is typically transferred to a cooling medium, such as cooling waters, remote from the area of energy input and in a form unsuitable for cycling. The less net energy requirement necessary to maintain a distillation process, the greater the efficiency of the process.
The use of solar energy for a significant portion of the net energy requirements reduces the energy input from other sources that can be costly and often limited in availability. Solar energy has not generally been used in distillation processes herebefore because of difficulties in efficient use of solar energy for such a purpose, i.e., difficulties in the build-up, storage, and transfer of the energy.
Energy is also generally required to condense the vapor. Energy is necessary at this stage at least to circulate the cooling medium. Rapid and effective condensation with minimum energy input at this stage of the distillation process increases the overall efficiency of the process.
Distillation processes additionally include those processes that separate one vaporizable liquid from another, and in such processes, commonly referred to as fractional distillations, efficient transportation of vapors to remote areas becomes a more significant aspect. (Fractional distillation processes depend on the phenomena that gaseous molecules move at rates dependent upon their molecular weights, other factors being the same. A gas molecule of a given molecular weight will move faster than a gas molecule of greater molecular weight.) Gas molecules, of course, will diffuse to occupy the entire available volume. In an air pressure gradient where space having a lesser density of gas molecules is available, a net movement of molecules across the gradient will occur until an equilibrium is reached. Many distillation processes depend mainly on the build-up of gas density at the area of vaporization to effectuate transportation of the vapor, i.e., diffusion of the vapor away from area of vaporization.